Magnetic data storage devices are commonly used in computers and network servers to store large quantities of digital information. Hard drives used in computers are a good example. Most such data storage devices have a number of flat, round rotating disks, and each disk has 2 surfaces which are coated with magnetic material. A read/write head is held in close proximity to each surface, enabling data to be transferred to and from the magnetic material. All the disks rotate at the same predetermined speed. The linear velocity of the magnetic material at the outer edge of the disks is greater than the linear velocity closer to the axis of rotation. This difference in linear velocities requires that each disk be divided into different zones arranged concentrically on the surface of each disk. Each zone has a different data rate at which data is read and written.
FIG. 1 illustrates the zoning scheme commonly used in magnetic data storage devices. A disk surface 20 is divided into concentric, circular zones (labeled 1-4) where a given zone has a number of individual data tracks 21. The zones are separated by zone boundaries 22. Each track in a given zone can be written to or read from at a certain data rate. Zones located nearer the periphery of the data surface have a higher data rate than the zones closer to the rotation axis. This feature results in the linear data density (the bits-per-inch, or BPI) for any given track being relatively constant over the entire surface of the disk. This maximizes the amount of data which can be stored on the disk.
In most disk drives, a computer accesses data in blocks of a fixed size (usually 512 bytes). When the computer sends a block of data to the drive, some error correction code or error detection bytes (ECC bytes) are usually added to the data block before the block is written onto the disk drive. Each data block has its own unique set of ECC bytes. ECC bytes can be used to detect or correct for errors when data is being read from the disk drive. Generally, more ECC bytes per data block results in lower error rates at the expense of data capacity.
Every disk surface and associated read/write head has a maximum data density (bits per square inch) which can be reliably stored and retrieved. This maximum data density is determined by the unique characteristics of a given disk surface/head pair. The maximum data density can be translated into a maximum data rate as a function of radial position 24. This is illustrated in the graph of FIG. 1. Some disk/head pairs are able to store data at a higher density than others due to variations in manufacturing processes. Attempting to store data at a higher density than what is possible on a disk surface results in high error rates during retrieval. If the data rates 26 in any of the zones exceeds the maximum data rate curve 24 for the disk, errors will be unacceptably high.
A typical hard drive has anywhere between 2-20 disk surfaces (1-10 disks). Since there exists a variation in the data capacity of each disk surface, more data can be stored more reliably if the data density stored on each surface is adjusted to be commensurate with the maximum data density of that surface. Disk surfaces capable of high data density (`warm` surfaces) store more data than surfaces not capable of high data density (`cold` surfaces). This is the general idea behind adaptive formatting of magnetic data storage devices. In the graph of FIG. 1, a warm surface will have a higher maximum data rate curve 24, and a cold surface will have a lower maximum data rate curve 24.
One way of changing the data capacity per surface is to displace the zone boundaries 22. Moving one or more zone boundaries 22 toward the rotation axis increases the amount of data stored on the surface, and moving one or more zone boundaries 22 away from the rotation axis will decrease the data stored. Altering the zone layout is just one is method of varying the data capacity of a data surface. Several other techniques are also known in the art.
FIG. 2 shows a side view of an adaptively formatted disk drive with 6 surfaces. The capacity of each surface is varied by changing only the zone layout (zone boundary locations) or each surface. Each surface has an adaptive format designed to maximize its data capacity while maintaining a minimum reliability. Due to variations in surface quality, each surface has a different zone layout.
Several problems associated with prior art adaptive formatting are as follows:
1) Adaptive formatting can result in hard drives having different total data capacities even though they are built with identical components. This is particularly undesired in OEM applications and in high performance computers and network servers. PA0 2) Adaptive formatting can require an increased number of transitions between different data rate zones and an increased number of long seek movements of the head transducers when reading or writing data. Both of these requirements slow the access time. This problem is at its worst when every surface in the drive has a different zone boundary layout. PA0 3) Adaptive formatting can result in the different data rate zones of a hard drive having different capacities. For example, a particular hard drive may be able to store 100 MB at the highest data rate, but an identical drive formatted differently may have only 90 MB of storage at the same data rate. This difference is plainly `visible` to circuitry external to the drive, which is undesirable. It is best for the data capacity of each data rate zone to be the same from drive to drive. PA0 4) Adaptive formatting can make hard drives more expensive due to the increased amount of time and testing necessary to measure the capacity of each data surface and the increased time necessary to custom format each data surface. PA0 1) results in total data capacity being the same from drive to drive; PA0 2) results in different drives having the same capacity at each data rate; PA0 3) minimizes the seek time and clock switches required for reading/writing data; PA0 4) minimizes the amount of address code processing which must be performed in order to read/write data; and PA0 5) increases the reliability and/or manufacturing yield of drives.
U.S. Pat. No. 5,087,992 to Dahandeh et al. describes a method of assigning zone boundaries on a data surface by measuring the error rates associated with different data rates. A zone boundary is identified as the radius at which the error rate exceeds a predetermined maximum. Hard drives formatted according to this adaptive method will suffer from all the problems listed above. The method of Dahandeh does not allow control of the data storage capacity of an entire drive. The method of Dahandeh is applicable only to single surfaces. Dahandeh does not teach a method for obtaining a drive with predetermined capacity characteristics.
U.S. Pat. No. 5,430,581 to Moribe et al. describes a method of formatting data surfaces which optimizes the linear data density of data tracks by first measuring a signal-to-noise figure for each track. Each track has a data density which is determined by the S-to-N figure. The zone boundary locations are determined by comparing the S-to-N figures to predetermined values. The method or Moribe also suffers from the problems listed above for adaptive formatting, and it is only applicable to individual data surfaces.
U.S. Pat. No. 5,537,264 to Pinteric discloses a method of maximizing data capacity in a drive by separating the magnetic heads used in the drive into high and low transfer rate (data rate) groups. Warm heads operate at a high data rate and cold heads operate at a lower data rate.
U.S. Pat. No. 5,596,458 to Emo et al. discloses a method of adaptive formatting wherein the zone boundaries on the different data surfaces have different locations. Unfortunately, using Emo's method will likely result in each data surface of a hard drive having a different zone layout. The data capacity of a hard drive built in accordance with Emo will likely be different from drive to drive. Also, Emo does not include provisions for minimizing the seek times required for reading/writing data. Reading and writing data in a device built according to Emo's invention will likely result in slow access times because each data surface may have a different zone layout. Further, the different zone layouts result in more complicated address conversion algorithms for organizing the data stored in the drive, increasing the cost of the hard drive. In short, a hard drive built in accordance with Emo's invention will be plagued with the problems listed above.